Fix `visualize_heap`

[hugsy]

Description/Motivation/Screenshots

Fixed several major bugs that prevented visualize-heap to work correctly.

Related issues/PRs:

• 76

• hugsy/gef#854

Depends on:

• hugsy/gef#869

cc @0xricksanchez

How Has This Been Tested ?

"Tested" indicates that the PR works and the unit test (i.e. make test) run passes without issue.

• [ ] x86-32
• [x] x86-64
• [ ] ARM
• [ ] AARCH64
• [ ] MIPS
• [ ] POWERPC
• [ ] SPARC
• [ ] RISC-V

Checklist

• [x] My PR was done against the dev branch, not main.
• [x] My code follows the code style of this project.
• [x] My change includes a change to the documentation, if required.
• [ ] If my change adds new code, adequate tests have been added.
• [x] I have read and agree to the CONTRIBUTING document.

[0xricksanchez]

Seems like something along the way of refactoring has gone missing when testing gef/heap_bugz with this branch here. For the exact setup refer to https://github.com/hugsy/gef/issues/854#issuecomment-1173467581

[hugsy]

Tests will fail until hugsy/gef#869 is merged.